Resolution enhancement device for an optically-coupled image sensor using high extra-mural absorbent fiber

ABSTRACT

A resolution enhancement device is provided which utilizes either high extra-mural absorbent optical fibers in the transfer optic, and/or which uses a transfer optic which is bonded to the scintillator without the use of any glues or adhesives. The device provides improved resolution of electron images from electron microscopes while not reducing the sensitivity of the apparatus.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/675,787, filed Sep. 29, 2000 now U.S. Pat. No. 6,455,860. The entiredisclosure of that application is hereby incorporated by reference. Thisapplication claims the benefit of U.S. provisional application SerialNo. 60/156,799, filed Sep. 30, 1999.

BACKGROUND OF THE INVENTION

The present invention relates to a resolution enhancement device thatmay be used, for example, in an electron microscope, and moreparticularly to the detection of electron images by converting them intolight images and transferring them onto an electronic light-imagingdevice to enhance the resolution of such images while not sacrificingsensitivity.

Electron microscopes use a beam of accelerated electrons that passthrough or are deflected by a sample to provide an electron image and/ordiffraction pattern of the sample. To provide a record of these imagesand/or diffraction patterns, the electrons have been converted intolight images using scintillator materials (e.g., single crystal YAG andphosphors), and an imaging sensor captures the light images and/orpatterns. A transfer optic, typically one or more optical lenses or afiber optic plate, transfers the light image to the imaging sensor.While photographic film and cameras have long been used to capture suchlight images and/or diffraction patterns, charge-coupled devices (CCD)of the type originally developed for astronomy to read light images intoa computer have found increasing use in this field. Such CCD camerasoffer excellent resolution, sensitivity, linearity, up to 2,048×2,048pixels, are reusable, and make the image available for viewing withinseconds of recording.

A conductive medium is typically coated onto the entrance surface of thescintillator to prevent the buildup of electrical charges and also toprevent the entry of light from external sources. When the transferoptic is a fiber optic plate, the scintillator is typically glued ontothe fiber optic plate, and the plate is then coupled with opticalcoupling oil or glue to the imaging sensor.

The resolution of prior art devices is limited by a number of factorsincluding the extent to which light generated at a particular spot onthe scintillator is imaged onto a single pixel at the imaging sensor.Current image coupling devices lose resolution due to leakage(scattering) of light sideways, either in the scintillator, the transferoptic, or both. Such light scattering increases background noise andcreates a “haze,” making it difficult to image objects generating onlyweak intensity light which are located near objects which generatestronger intensity light.

One solution to the problem of image resolution is suggested by Mooneyet al, U.S. Pat. No. 5,635,720. There, a light absorptive layer ispositioned on a scintillator to absorb reflected, scattered light fromthe scintillator and prevent that scattered light from reaching theimaging device. However, while improving resolution of the image, thesensitivity of the device is reduced because of the absorption of light.

Extra mural absorption materials such as glasses have been used toabsorb stray light from optical fibers. However, typically, extra muralabsorption glass is introduced as separate fibers into a fiber bundle(for example, every n^(th) fiber is a substituted EMA fiber, where n isa number >>1). Such technique provides a statistical level of absorbedlight, typically less than 10%, but does not provide the type ofselective light absorption required in electron microscopy.

Accordingly, the need still exists in the art for an apparatus that willimprove the ultimate resolution of image sensors used to record imageswhile not reducing the sensitivity of the apparatus.

SUMMARY OF THE INVENTION

The present invention meets that need by providing a resolutionenhancement device which utilizes either high extra-mural absorbentoptical fibers in the transfer optic, and/or which uses a transfer opticwhich is bonded to the scintillator without the use of any externalglues or adhesives. The device provides improved resolution of electronimages, such as for example from electron microscopes, while notreducing the sensitivity of the apparatus. This enables the device to beused to observe and image objects that generate only weak lightintensity but which are positioned near other objects which generatestronger light intensity.

In accordance with one embodiment of the invention, a resolutionenhancement device is provided and includes an imaging sensor adapted toreceive and record a light image, a scintillator adapted to convert anelectron image into a light image, and a transfer optic associated withthe scintillator and the imaging sensor for transferring the light imagefrom the scintillator to the imaging sensor. The transfer opticcomprises at least one optical fiber including a layer of claddingmaterial, the at least one optical fiber being oriented lengthwise withrespect to an optical axis of the device. The at least one optical fiberincludes a layer of light absorptive material on the layer of claddingmaterial which attenuates at least a portion of off-axis light enteringthe transfer optic. By “off-axis” light, it is meant light that entersthe transfer optic at an angle greater than the critical angle.Preferably, the transfer optic comprises multiple optical fibers packedin an array, typically a hexagonal array.

In a preferred embodiment of the invention, the scintillator comprises alayer that includes a first surface for receiving an electron image anda second surface adjacent the transfer optic. The resolution enhancementdevice includes a light reflective layer positioned on the first surfaceof the scintillator. The imaging sensor is preferably a charge-coupleddevice and the transfer optic is a fiber optic plate. The scintillatormay be any scintillator material that has found use in this artincluding single crystal yttrium-aluminum-garnet as well as coatings ofparticulate phosphors. Preferably, the cladding material and the lightabsorptive layer have a difference in refractive indices of less thanabout 0.1.

A preferred environment for the present invention is in an electronmicroscope having a projection chamber through which an electron beamforming an electron image and/or diffraction pattern traverse. Such anapparatus includes a scintillator located in the path of the electronbeam for converting the electron image into a light image, and animaging sensor positioned to receive and record the light image. Theapparatus further includes the transfer optic associated with thescintillator for transferring the optical image to the imaging sensor asdescribed previously.

In a preferred embodiment, the transfer optic and the scintillator arebonded to one another in the absence of a bonding agent such as a glueor other adhesive. Such a glue layer increases light scattering due torefractive index mismatches at the scintillator/glue and glue/transferoptic interfaces. The present invention eliminates such a glue layer andinstead directly bonds the scintillator and transfer optic to oneanother. In a preferred form, the transfer optic and the scintillatorare bonded using optical contacting of the respective surfaces followedby heat treatment to form a virtually defect-free bond interface withoutthe need for glues or other bonding agents. Also, preferably, thetransfer optic and the scintillator have refractive indices that differby less than about 0.1. This embodiment also finds use in an electronmicroscope.

In a further embodiment of the invention, a resolution enhancementdevice is provided and includes an imaging sensor adapted to receive andrecord a light image, a scintillator adapted to convert an electronimage into a light image, and a transfer optic associated with thescintillator and the imaging sensor for transferring the light imagefrom the scintillator to the imaging sensor. The transfer optic and thescintillator are bonded together by optical contacting and subsequentheat treatment. The transfer optic comprises at least one optical fiberincluding a layer of cladding material, with the at least one opticalfiber being oriented lengthwise with respect to an optical axis of thedevice. The at least one optical fiber includes a layer of lightabsorptive material on the layer of cladding material which attenuatesat least a portion of off-axis light entering the transfer optic.

In yet another embodiment, an apparatus for improving the resolution ofelectron images is provided and includes an electron beam forming anelectron image and a scintillator located in the path of the electronbeam for converting the electron image into a light image. A lightreflective layer is positioned between the scintillator and the sourceof the electron beam, and an imaging sensor is positioned to receive andrecord the light image. The apparatus further includes a transfer opticassociated with the scintillator for transferring the optical image tothe imaging sensor, the transfer optic and the scintillator being bondedto one another in the absence of a bonding agent. The transfer opticcomprises at least one optical fiber including a layer of claddingmaterial, the at least one optical fiber is oriented lengthwise withrespect to an optical axis of the apparatus. The at least one opticalfiber includes a layer of light absorptive material on the layer ofcladding material which attenuates at least a portion of off-axis lightentering the transfer optic. Preferably, the light reflective layercomprises an electrically conductive material such as, for example,aluminum.

In yet a further embodiment, an electron microscope is provided andincludes a projection chamber through which an electron beam forming anelectron image and/or diffraction pattern traverses. The microscopeincludes an apparatus for improving the resolution of images produced bythe electron microscope and is positioned in the projection chamber tointercept the electron beam. The resolution enhancement apparatusincludes a scintillator located in the path of the electron beam forconverting the electron image into a light image, a light reflectivelayer positioned between the scintillator and the source of the electronbeam, and an imaging sensor positioned to receive and record the lightimage. The apparatus further includes a transfer optic associated withthe scintillator for transferring the optical image to the imagingsensor. The transfer optic comprises at least one optical fiberincluding a layer of cladding material, the at least one optical fiberis oriented lengthwise with respect to an optical axis of the apparatusand includes a layer of light absorptive material on the layer ofcladding material which attenuates at least a portion of off-axis lightentering the transfer optic.

Accordingly, it is a feature of the present invention to provideresolution enhancement for image sensors that are used to record imagesfrom electron microscopes while not compromising or degrading thesensitivity of the apparatus. This, and other features and advantages ofthe present invention, will become apparent from the following detaileddescription, the accompanying drawings, and the appended claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged schematic cross-sectional view of the apparatus ofthe present invention;

FIG. 2 is a schematic cross-sectional view of the apparatus of thepresent invention positioned in the projection chamber of an electronmicroscope;

FIG. 3 is a cross-sectional view of an exemplary optical fiber,including cladding and EMA layer;

FIG. 4 is a cross-sectional view of a portion of a hexagonally stackedbundle of optical fibers; and

FIG. 5 is an enlarged cross-sectional view of a portion of a hexagonallypacked bundle of optical fibers that have been stacked, pressed, anddrawn.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the resolution enhancement apparatus of the presentinvention is illustrated. The resolution enhancement device, generallyindicated at 10, includes scintillator 12 supported by a transfer opticsuch as fiber optic plate 14. In a preferred form scintillator 12comprises a single crystal such as a yttrium-aluminum-garnet crystal.Such crystals can be manufactured to a thickness of from about 5 to 50μm. However, it should be recognized that other scintillators may beused including polycrystalline materials and particulate materials. Forexample, a powdered phosphor may be coated onto the fiber optic plate toform the scintillator. Generally, particle sizes for the phosphor shouldbe 1 μm or less, and the coating applied to a thickness of between about1 to 25 μm to minimize random electron and light scatter from theparticles.

As the transfer optic, a fiber optic plate is provided which includesstacked glass fibers which are clad on their surfaces with a claddingglass or other material. Additionally, in the present invention, a layerof light absorptive material overlies the layer of cladding. FIGS. 3-5(which are not drawn to scale) schematically illustrate, incross-section, the general construction of such optical fibers. There,an optical fiber core 100 has a layer of cladding material 102 thereon.Preferably, optical fiber 100 is a glass having a relatively highrefractive index (typically at least about 1.8). This provides arelatively close match with the refractive index of single crystal YAGscintillators (RI approx. 1.83). Preferably, the difference inrefractive indices between the core glass and the scintillator is lessthan about 0.1. This reduces light refraction and scattering at theoptical fiber/scintillator interface. Moreover, the relatively highrefractive index of the core glass provides the refractive indexmismatch needed with the cladding layer to provide a high numericalaperture fiber (≧0.8). Numerical aperture is defined as the sine of thehalf angle at which light (in air) enters a fiber, propagates along itslength, and exits at the other end. Numerical aperture is a function ofthe refractive indices of the core glass and cladding material. Claddinglayer 102 is also preferably a glass such as a boro-silicate glass.Typically, cladding glass 102 has a refractive index of about 1.5.

Overlying cladding material 102 is a layer of a light (optically)absorptive material 104. Preferably, light absorptive material 104 is ahigh extra mural material such as a dark colored glass (most preferablyblack) that is designed to substantially attenuate any high angle,off-axis light which may be scattered from adjacent optical fibers. Suchglasses are also typically boro-silicate glasses having refractiveindices of approximately 1.5. The glasses may be colored usingtechniques which are known, such as, for example, adding small amountsof certain metals to the glass composition.

Generally, the cladding material 102 and the light absorptive material104 have slightly lower melting points than the core glass fiber so thata fiber bundle can be formed and then heated to soften the cladding andlight absorptive layers to produce a fused fiber bundle. As shownschematically in FIG. 4, groups of optical fibers are gathered togetherto form, typically, hexagonally stacked bundles. Upon heating, thebundles of fibers may be drawn and pressed so that the bundles assume amore hexagonal shape as shown in FIG. 5. The pressed, drawn, and stackedfiber bundles are then consolidated into a fiber optic plate 14. Thefiber bundles may also be cut one or more times during fabrication. Asshown, the fibers in the fiber optic plates are oriented lengthwise withrespect to an optical axis of the device. Typically, such fiber opticplates have a thickness on the order of a few millimeters and diametersup to about 40 mm. Individual fibers have a diameter of typically lessthan about 10 μm. The fiber optic plate thus has parallel orientedfibers that transmit light in an ordered fashion such that an image atone end is transferred fiber by fiber (pixel by pixel) to the other end.

Referring back again to FIG. 2, scintillator 12 optionally includesthereon a light reflective layer 16 which acts to reflect scatteredlight back through the transfer optic to improve the sensitivity of theapparatus. Reflective layer 16 may comprise a thin layer of a conductivemetal such as aluminum that is transparent to electrons. The reflectivelayer thus may also perform the function of preventing the buildup ofelectrical charges on the apparatus which could discharge and causearcing. Additionally, light reflective layer 16 is opaque to externallight sources and prevents such light from entering the apparatus.

To further improve the resolution capabilities and sensitivity of theapparatus, scintillator 12 and fiber optic plate 14 are secured togetherwithout the use of a conventional bonding agent such as an adhesive orglue. Rather, scintillator 12 and fiber optic plate 14 are bondedthrough the use of an optical contacting technique and subsequent heattreatment as taught by Meissner, U.S. Pat. No. 5,846,638, the subjectmatter of which is incorporated by reference herein.

It is desirable that the transfer optic transfers the maximum amount oflight from the scintillator to the imaging sensor. With a glue or otherbonding agent, there will be some portion of the light which isreflected at each interface (scintillator/glue and glue/transfer optic)resulting in an increase in background “noise” (i.e., stray light).Providing a bonding agent-free interface increases the sensitivity ofthe apparatus while also improving resolution because there is lessreflection and light scatter.

When combined with the light absorptive extra mural layer, high anglelight entering each optical fiber and light not hitting the center ofthe fiber (and which gets scattered as high angle light) aresubstantially attenuated (absorbed), thereby also improving imageresolution. High angle light is light that enters the transfer optic atan angle greater than the critical angle.

Referring back to FIG. 1, fiber optic plate 14 may be optically coupledto an imaging sensor 20 using a fluid oil 18. Such an oil, selected tohave an index of refraction the same as or very close to that of theglass in fiber optic plate 14, improves the transmission of lightbetween fiber optic plate 14 and imaging sensor 20. Further, as imagingsensor 20 will be operated at relatively low temperatures, as describedin greater detail below, the coupling oil should have a low freezingpoint so that it will remain fluid at temperatures down to about −40° C.

Referring now to FIG. 2, a schematic view of a typical use of thepresent invention is shown in which an imaging device such as acharge-coupled device (CCD) camera 40 is mounted on the projectionchamber 42 of a transmission electron microscope (TEM). As will beappreciated, the apparatus of the present invention may also find use ina scanning electron microscope (SEM), or a scanning, transmissionelectron microscope (STEM) as well. Typically, the projection chamber isattached to the end of an optical column of a TEM and houses a viewingscreen 44 which is either lowered into an observation position or raisedinto a position in which it does not intercept electron beam 46 that isprojected into the chamber. The projection chamber may also house a filmmagazine comprising a transport mechanism (not shown) which inserts asheet of photographic film 48 into an exposure position and returns thesheet into the magazine after exposure.

The typical projection chamber further has several ports suitable forattaching an imaging device such as a camera, one of which is usuallysituated at the bottom of the chamber. The chamber is normally evacuatedvia a vacuum pipe 50 leading to a gate valve 52 which can either open orclose the chamber to a high vacuum (e.g., 10⁻⁶ torr) pump 54. The gatevalve in most modern TEMs is controlled pneumatically via two inlets 56and 58 such that introduction of pressurized air into one inlet causesthe valve to open, and the introduction of pressurized air into theother inlet causes the valve to close.

An electron beam 46 forming an electron image or diffraction patternfrom a specimen in the microscope traverses the projection chamber 42.Camera 40 includes resolution enhancement device 10 (shown in enlargedcross-section in FIG. 1). Device 10 includes a scintillator 12 thatconverts the electron image into a light image. Scintillator 12 issupported on a transfer optic such as fiber optic plate 14. By lightimage, it is generally meant light in the visible spectrum, althoughthere are some scintillation materials that can produce light outside ofthe visible spectrum in either the near infrared or in the ultravioletregions of the spectrum. It is within the scope of the present inventionto use scintillator materials that produce images in the infrared,visible, and/or ultraviolet portion of the spectrum.

Fiber optic plate 14 is optically coupled to an imaging sensor such as atwo-dimensional charge-coupled device (CCD) sensor 20 with anoptically-coupling oil layer 18. Such CCD sensors are commerciallyavailable from several manufacturers including Kodak, Ford, ScientificImaging Technologies (SITe), Hamamatsu, Thomson CSF, and EnglishElectric Valve Ltd. Preferred solid-state imaging devices are scientificgrade CCDs whose imaging areas comprise 1024×1024 or more pixels.However, it should be appreciated that any imaging device that iscapable of capturing a light image and producing an electronic signalmay be utilized including a cathode ray television tube.

The preferred CCD should be operated cold to keep its dark current smallenough so that the noise in the dark current accumulated during atypical exposure does not limit the performance of the camera. Thetypical exposure in an electron microscope is from about 1 to 20seconds. Maintaining the CCD at a temperature of about −25° to about−40° C. is typically sufficiently low for the accumulated dark currentto be acceptably small at exposure times of up to about 1 minute. Such atemperature is conventionally achieved using a thermoelectric coolingdevice (not shown), whose cold side may be in contact with the imagingsensor 20.

The CCD is connected to an external electronics unit 60 through a vacuumfeed-through 62 that transfers the captured images to the memory of adigital computer 64. The images may be displayed on a view screen 66,such as a CRT, attached to the computer. For the present invention, theimages may be digitized and then displayed using Digital/Micrographsoftware commercially available from Gatan, Inc., Pleasanton, Calif.Other details of operation of the apparatus are set forth incommonly-owned U.S. Pat. Nos. 5,635,720 and 5,065,029, the disclosuresof which are incorporated by reference.

Referring again to FIG. 2, in operation, an electron image ordiffraction pattern 46 from a sample (not shown) traverses projectionchamber 42 and impinges on scintillator 12. Electrons in the beam thatcollide with the scintillation material produce corresponding lightphotons that travel towards fiber optic plate 14. Light which israndomly scattered, reflected internally within scintillator 12, ordeflected laterally and reflected is substantially attenuated by lightabsorptive layers 104 on the optical fibers.

The light image is then directed to imaging sensor 20 through thetransfer optic, fiber optic plate 14. Once the image impinges on sensor20, it is detected and then displayed on view screen 66 of digitalcomputer 64. Because substantially all of the laterally scattered lighthas been absorbed previously, the image resolution is enhanced.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the methods and apparatusdisclosed herein may be made without departing from the scope of theinvention, which is defined in the appended claims.

What is claimed is:
 1. A resolution enhancement device comprising: animaging sensor adapted to receive and record a light image, ascintillator adapted to convert an electron image into a light image,and a transfer optic associated with said scintillator and said imagingsensor for transferring said light image from said scintillator to saidimaging sensor, said transfer optic comprising at least one opticalfiber including a layer of cladding material, said at least one opticalfiber being oriented lengthwise with respect to an optical axis of saiddevice, said at least one optical fiber including a layer of lightabsorptive material on said layer of cladding material which attenuatesat least a portion of off-axis light entering said transfer optic.
 2. Adevice as claimed in claim 1 wherein said scintillator comprises a layerthat includes a first surface for receiving an electron image and asecond surface adjacent said transfer optic, said device including alight reflective layer positioned on said first surface of saidscintillator.
 3. A device as claimed in claim 2 in which said lightreflective layer comprises an electrically conductive material.
 4. Adevice as claimed in claim 1 in which said transfer optic and saidscintillator are bonded to one another in the absence of a bondingagent.
 5. A device as claimed in claim 1 in which said imaging sensorcomprises a charge-coupled device.
 6. A device as claimed in claim 1 inwhich said transfer optic comprises a fiber optic plate.
 7. A device asclaimed in claim 1 in which said scintillator comprises ayttrium-aluminum-garnet crystal.
 8. A device as claimed in claim 1 inwhich said scintillator comprises a coating of a particulate phosphor.9. A device as claimed in claim 1 in which said cladding material andsaid light absorptive layer have a difference in refractive indices ofless than about 0.1.
 10. A device as claimed in claim 1 in which saidtransfer optic comprises multiple optical fibers packed in a hexagonalarray.
 11. A resolution enhancement device comprising: an imaging sensoradapted to receive and record a light image, a scintillator adapted toconvert an electron image into a light image, and a transfer opticassociated with said scintillator and said imaging sensor fortransferring said light image from said scintillator to said imagingsensor, said transfer optic and said scintillator being bonded byoptical contacting and subsequent heat treatment, said transfer opticcomprising at least one optical fiber including a layer of claddingmaterial, said at least one optical fiber being oriented lengthwise withrespect to an optical axis of said device, said at least one opticalfiber including a layer of light absorptive material on said layer ofcladding material which attenuates at least a portion of off-axis lightentering said transfer optic.
 12. An apparatus for improving theresolution of electron images comprising: an electron beam forming anelectron image, a scintillator located in the path of said electron beamfor converting said electron image into a light image, a lightreflective layer positioned between said scintillator and the source ofsaid electron beam, and an imaging sensor positioned to receive andrecord said light image, said apparatus further including a transferoptic associated with said scintillator for transferring said opticalimage to said imaging sensor, said transfer optic and said scintillatorbeing bonded to one another in the absence of a bonding agent, saidtransfer optic comprising at least one optical fiber including a layerof cladding material, said at least one optical fiber being orientedlengthwise with respect to an optical axis of said apparatus, said atleast one optical fiber including a layer of light absorptive materialon said layer of cladding material which attenuates at least a portionof off-axis light entering said transfer optic.
 13. An apparatus asclaimed in claim 12 wherein said light reflective layer comprises anelectrically conductive material.
 14. An apparatus as claimed in claim13 in which said light reflective layer comprises aluminum.
 15. In anelectron microscope having a projection chamber through which anelectron beam forming an electron image and/or diffraction patterntraverses, an apparatus for improving the resolution of images producedby said electron microscope and positioned in said projection chamber tointercept said electron beam comprising: a scintillator located in thepath of said electron beam for converting said electron image into alight image, a light reflective layer positioned between saidscintillator and the source of said electron beam, and an imaging sensorpositioned to receive and record said light image, said apparatusfurther including a transfer optic associated with said scintillator fortransferring said optical image to said imaging sensor, said transferoptic comprising at least one optical fiber including a layer ofcladding material, said at least one optical fiber being orientedlengthwise with respect to an optical axis of said apparatus, said atleast one optical fiber including a layer of light absorptive materialon said layer of cladding material which attenuates at least a portionof off-axis light entering said transfer optic.
 16. An apparatus asclaimed in claim 15 wherein said light reflective layer comprises anelectrically conductive material.
 17. An apparatus for improving theresolution of electron images consisting of: an electron beam forming anelectron image, a scintillator located in the path of said electron beamfor converting said electron image into a light image and an imagingsensor positioned to receive and record said light image, said apparatusfurther including a transfer optic associated with said scintillator fortransferring said optical image to said imaging sensor, said transferoptic comprising at least one optical fiber including a layer ofcladding material, said at least one optical fiber being orientedlengthwise with respect to an optical axis of said apparatus, said atleast one optical fiber including a layer of light absorptive materialon said layer of cladding material which attenuates at least a portionof off-axis light entering said transfer optic.